Offshore platform subsidence remediation: Synchronous deck-raising solution

December 1, 2014

Off the coast of Jakarta, Indonesia, a bridge-linked platform complex was raised 4m in order to increase the air gap on three platforms and their supporting structures.

The LIMA Flow Station, operated by PT. PERTAMINA Hulu Energi Offshore North West Java (PHE ONWJ), experienced a loss of air gap due to the ocean floor subsidence that occurred in the surrounding seabed over time. PHE ONWJ is an oil & gas contractor working under a production-sharing contract (PSC). It’s a subsidiary of the national oil & gas company, PT PERTAMINA (Persero), which is operating the Offshore North West Java (ONWJ) field after assuming ownership from BP West Java in July 2009. 

Aerial view of LIMA flow station. Photos from Pertamina Hulu Energi Offshore North West Java (PHE ONWJ).
 

Subsidence and air gap reduction

Over the life of a production facility, reservoir pressure will decrease as oil and gas flow to the surface. This reduction in reservoir pressure may result in local compaction and consolidation of reservoir rock. Depending upon the local geology, such reservoir compaction may also lead to subsidence at the surface, or at the seabed in the case of an offshore platform.

For a fixed offshore platform, a consequence of seabed subsidence is the reduction of the air gap between average sea level and the underside of the topside, increasing the potential for inundation of the deck in extreme storms. This will put the platform at risk for structural failure under storm loading conditions. As the probability of extreme waves entering the deck increases with further subsidence, some form of remediation is generally required to ensure that the offshore platform continues to maintain satisfactory levels of structural reliability.

Two-stage rams deployed during synchronous lift at LIMA flow station.
 

Platform remediation

The LIMA flow station was built in 1973 by Atlantic Richfield Indonesia Inc. It consists of a production platform, a compression platform, and a living quarters platform, with three bridges and flare support structures.

Although the LIMA flow station was operating normally without remediation, platform safety could become compromised while its production is still attractive. PHE ONWJ had thoroughly evaluated various options to remedy the flow station to safe operation until the projected end of production life in 2026. The options included: do nothing, conventional lifting, ON-OFF operation, build new platform(s), MOPU, flat-over barge for deck raising, and deck raising by hydraulic jacking system.

Additional criteria, technical feasibility, commercial matters, period of shutdown need, project schedule, and risks had to be taken into consideration during the evaluation. The evaluation concluded that deck raising by hydraulic jacking system offered the best cost-effective and technically sound solution to restore a safe distance between each platform’s cellar deck and the mean sea level.

Deck-raising solution

Flare bridge with lift equipment at LIMA flow station. 
 

Versabar’s patented deck-raising system was selected to raise the LIMA Flow Station simultaneously, and turned into the world’s first application of a synchronized hydraulic jacking system with PLC control, combined with encapsulated leg-sleeves used for deck raising.

PHE ONWJ’s “Lima Subsidence Remediation Project” awarded two contracts, one to PT. SAS International to provide all specialty deck-raising equipment and services, the second to PT. Timas Suplindo (Timas) as the EPCI (engineering, procurement, construction and installation) contractor. Versabar was subcontracted by PT. SAS International to furnish all synchronized hydraulic jacking equipment.

Versabar’s deck raising technology was first developed in 2006 when a Gulf of Mexico operator needed to raise the topsides on two 8-leg drilling and production platforms. Using hydraulically and electronically synchronized rams, each platform was successfully elevated 14 ft in less than two hours.

This was a unique solution which uses split sleeves to encapsulate the legs throughout the raising process. This encapsulation provides a high degree of lateral stability during the raising operation and subsequently forms the permanent leg extensions for the raised condition.

Versabar provided detailed engineering, fabrication, and testing of all deck-raising equipment, leg extension sleeves, and other raising appurtenances. Two types of rams were custom engineered to raise the platforms in two stages, and a third type of ram was used to simultaneously raise the supporting bridge structures for the full 4m. During the deck raising procedure, the Stage I rams raise the platforms 38.5 inches (in.), and then leg pins are inserted. In Stage II, dual-rod rams raise the platforms to 106in. After pinning off, the Stage II rams continue to raise the platforms to 159.5in., and then pin off once more. 

The custom engineered HPUs (hydraulic power units) that operate the rams incorporate a redundant design using two of everything where applicable, starting with the utilization of twin 200 HP diesel engine and hydraulic pump arrangements, each with their own fuel supply and cooling capability. In the unlikely event of an engine or pump failure during the raising operation, the units were designed to be able to continue the raising process unaffected. The HPUs also integrated a fail-safe design into the control scheme so that if there is a lost control or hydraulic signal, all operations would automatically halt until the matter was resolved.

Testing

Leg sleeves at LIMA flow station.
 

Prior to mobilizing offshore, Versabar built a full-scale detailed deck and bridge construction mirroring one of the platforms in order to perform an accurate System integration test (SIT) at the Versabar facility in Houston. Each stage of the lift was performed, with split sleeves being installed on the legs, and leg pins being inserted at each point of the raising operation as they would be offshore. The mock-up topside and connecting bridge were raised the full 4m.

According to Versabar Project Engineer, Sid Fabre, “Our stringent test acceptance criterion ensures the system is proven and job ready. During the testing of the complete deck-raising system, we went above full working load to demonstrate the capacity of the system. We also went through a sequence of simulated malfunctions and failure mode testing under load to evaluate the failsafe design and redundancy of the hydraulic and control system.”

Transport

All equipment was shipped from Houston to Indonesia using project cargo transportation. This included six, 400 HP hydraulic power units weighing 20 tons each, one control cabin, and 48 40ft shipping containers with a total of 120 rams (12 rams for the connecting bridges, 48 Stage 1 rams, 48 Stage 2 rams, plus spares). Extra care and preparation was taken to ensure all cargo and components arrived safely and on schedule.

Offshore Lift

In preparation for the deck raise, Timas prepared the platforms by installing all of the Versabar provided equipment on the topsides and bridges, cutting and capping the risers and appurtenances, installing the phase 1 sleeves and bushings on the deck legs and finally installing the phase 1 raising hydraulic rams. Versabar personnel arrived onsite on 2 August 2013 to perform offshore installation, hook-up and commissioning of the deck raising system, and operation of the raising equipment for the actual lift.

The raising operation began on 4 September. All six structures, with a combined weight of approximately 3300 tons, were simultaneously raised in parallel 38.5in. to complete Stage I of the lift in 2.25 hr. Fifteen days later, after the Stage I rams were removed and Stage II leg extension sleeves and rams were installed, the second phase of the lift took place. Once the structures were lifted to 106in., the Stage II intermediate pin was inserted, and then raising continued to the full height of 159.5in. The legs were then pinned again, providing a full mechanical lock-off of the deck in the raised condition. Stage II was performed in 4 hr, which helped play a part in reaching the Lima Subsidence Remediation Project’s ultimate goal, which was not only to raise the decks, but most importantly, to get the facility back in production on schedule. 

According to Versabar’s Project Manager Sid Fabre, “During the design and testing phases of the project we left no stone unturned. We executed high-level commissioning checklists to verify all structural, mechanical, electrical and operational aspects of the deck raising system during installation and the actual raising operation. In addition, a good working relationship and quality installation of deck raising components by EPCI contractor TIMAS played a critical role. We at Versabar believe in engineered solutions, testing, testing and more testing. When installing and commissioning a system for an operation this critical, preparation provides the best opportunity for a safe, problem-free operation.” It is projected that the increased air gap provided by the deck raise will grant an additional 12-15 years of use of the Lima Flow Station for oil & gas production from the PNWJ field.

On this remarkable achievement, Jonly Sinulingga, Executive VP/General Manger PHE ONWJ said, “PHE ONWJ is proud to complete this project safely and timely. This raising of three platforms, three bridges and flare support structures at once with a synchronized jacking system simultaneously is the first achievement in PHE ONWJ, Pertamina, Indonesia and probably the world. The success of this project has given us the experience and confidence to apply the most advanced technology application in our offshore operations.”



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